Terry Pratchett - The Science of Discworld

by Terry Pratchett

  Unnatural selection was a fact, but the wizards knew, they knew, that you couldn’t start off with bananas and get fish.

  The Librarian glanced at the card, and took a few surprising turnings. There was the occasional burst of noise on the other side of the shelves, rapidly changing as though someone was playing with handfuls of sound, and a flickering in the air. Someone talking was replaced with the absorbent silence of empty rooms was replaced with the crackling of flame and displaced by laughter ...

  Eventually, after much walking and climbing, the Librarian was faced with a blank wall of books. He stepped up to them with librarianic confidence and they melted away in front of him.

  He was in some sort of study. It was book-lined, although with rather fewer than the Librarian would have expected to find in such an important node of L-space. Perhaps there was just the one book ... and there it was, giving out L-radiation at a strength the Librarian had seldom encountered outside the seriously magical books in the locked cellars of Unseen University. It was a book and father of books, the progenitor of a whole race that would flutter down the centuries ...

  It was also, unfortunately, still being written.

  The author, pen still in hand, was staring at the Librarian as if he’d seen a ghost.

  With the exception of his bald head and a beard that even a wiz­ard would envy, he looked very, very much like the Librarian.

  ’My goodness ...’

  ’Ook?’ The Librarian had not expected to be seen. The writer must have something very pertinent on his mind.

  ’What manner of shade are you ... ?’

  ’Ook.’[36]

  A hand reached out, tremulously. Feeling that something was expected of him, the Librarian reached out as well, and the tips of the fingers touched.

  The author blinked.

  ’Tell me, then,’ he said, ’is Man an ape, or is he an angel?’

  The Librarian knew this one.

  ’Ook,’ he said, which meant: ape is best, because you don’t have to fly and you’re allowed sex, unless you work at Unseen University, worst luck.

  Then he backed away hurriedly, ooking apologetic noises about the minor error in the spacetime coordinates, and knuckled off through the interstices of L-space and grabbed the first book he found that had the word ’Evolution’ in the title.

  The bearded man went on to write an even more amazing book. If only he had thought to use the word ’Ascent’ there might not have been all that unpleasantness.

  But, there again, perhaps not.

  HEX let itself absorb more of the future ... call it ... knowledge. Words were so difficult. Everything was context. There was too much to learn. It was like trying to understand a giant machine when you didn’t understand a screwdriver.

  Sometimes HEX thought it was picking up fragmentary instruc­tions. And, further away, much further away, there were little disjointed phrases in the soup of concepts which made sense but did not seem to be sensible. Some of them arrived unbidden.

  Even as HEX pondered this, another one arrived and offered an opportunity to make $$$$ While You Sit On Your Butt!!!!! He con­sidered this unlikely.

  The title brought back by the Librarian was The Young Person’s Guide to Evolution.

  The Archchancellor turned the pages carefully. They were well illustrated. The Librarian knew his wizards.

  ’And this is a good book on evolution?’ said the Archchancellor.

  ’Ook.’

  ’Well, it makes no sense to me,’ said the Archchancellor. ’I mean t’say, what the hell is this picture all about?’

  It showed, on the left, a rather hunched-up, ape-like figure. As it crossed the page, it gradually arose and grew considerably less hairy until it was striding confidently towards the edge of the page, per­haps pleased that it had essayed this perilous journey without at any time showing its genitals.

  ’Looks like me when I’m getting up in the mornings,’ said the Dean, who was reading over his shoulder.

  ’Where’d the hair go?’ Ridcully demanded.

  ’Well, some people shave,’ said the Dean.

  ’This is a very strange book,’ said Ridcully, looking accusingly at the Librarian, who kept quiet because in fact he was a little worried. He rather suspected he might have altered history, or at least a his­tory, and on his flight back to the safety of UU he’d seized the first book that looked as though it might be suitable for people with a very high IQ but a mental age of about ten. It had been in an empty byway, far off his usual planes of exploration, and there had been very small red chairs in it.

  ’Oh, I get it. This is a fairy story,’ said Ridcully. ’Frogs turnin’ into princes, that kind of thing. See here ... there’s something like our blobs, and then these fishes, and then it’s a ... a newt, and then it’s a big dragony type of thing and, hah, then it’s a mouse, then here’s an ape, and then it’s a man. This sort of thing happens all the time out in the really rural areas, you know, where some of the witches can be quite vindictive.’

  ’The Omnians believe something like this, you know,’ said the Senior Wrangler. ’Om started off making simple things like snakes, they say, and worked his way up to Man.’

  ’As if life was like modelling clay?’ said Ridcully, who was not a patient man with religion. ’You start out with simple things and then progress to elephants and birds which don’t stand up properly when you put them down? We’ve met the God of Evolution, gentle­men ... remember? Natural evolution merely improves a species. It can’t change anything.’

  His finger stabbed at the next page in the brightly coloured book.

  ’Gentlemen, this is merely some sort of book of magic, possibly about the Morphic Bounce Hypothesis.[37] Look at this.’ The picture showed a very large lizard followed by a big red arrow, followed by a bird. ’Lizards don’t turn into birds. If they did, why have we still got lizards? Things can’t decide for themselves what shape they’re going to be. Ain’t that so, Bursar?’

  The Bursar nodded happily. He was halfway through HEX’s write-out of the theoretical physics of the project universe and, so far, had understood every word. He was particular happy with the limitations of light speed. It made absolute sense.

  He took a crayon and wrote in the margin: ’Assuming the uni­verse to be a negatively curved non-Paramidean manifold - which is more or less obvious - you could deduce its topology by observing the same galaxies in several different directions.’ He thought for a moment, and added: ’Some travel will be involved.’

  Of course, he was a natural mathematician, and one thing a nat­ural mathematician wants to do is get away from actual damn sums as quickly as possible and slide into those bright sunny uplands where everything is explained by letters in a foreign alphabet, and no one shouts very much. This was even better than that. The hard-to-digest idea that there were dozens of dimensions rolled up where you couldn’t see them was sheer jelly and ice cream to a man who saw lots of things no one else saw.

  TWENTY-SIX

  THE DESCENT OF DARWIN

  THE WIZARDS MET THE GOD OF EVOLUTION in The Last Continent. He made things the way a god ought to:

  "’Amazin’ piece of work," said Ridcully,

  emerging from the elephant. "Very good wheels. You paint these bits before assembly, do you?"’

  The God of Evolution builds creatures piece by piece, like a butcher in reverse. He likes worms and snakes because they’re very easy - you can roll them out like a child with modelling clay. But once the God of Evolution has made a species, can it change? It does on Discworld, because the God runs around making hurried adjustments . . . but how does it work without such divine interven­tion?

  All societies that have domestic animals, be they hunting dogs or edible pigs, know that living creatures can undergo gradual changes in form from one generation to the next. Human intervention, in the form of ’unnatural selection’, can breed long thin dogs to go down holes and big fat pigs that provide more bacon per trotter.[38] The wizards kno
w this, and so did the Victorians. Until the nine­teenth century, though, nobody seems to have realized that a very similar process might explain the remarkable diversity of life on Earth, from bacteria to bactrians, from oranges to orangutans.

  They didn’t appreciate that possibility for two reasons. When you bred dogs, what you got was a different kind of dog - not a banana or a fish. And breeding animals was the purest kind of magic: if a human being wanted a long thin dog, and if they started from short fat ones, and if they knew how the trick worked (if, so to speak, they cast the right ’spells’) then they would get a long thin dog. Bananas, long and thin though they might be, were not a good starting point. Organisms couldn’t change species, and they only changed form within their own species because people wanted them to.

  Around 1850, two people independently began to wonder whether nature might play a similar game, but on a much longer timescale and in a much grander manner - and without any sense of purpose or goal (which had been the flaw in previous musings along similar lines). They considered a self-propelled magic: ’natural’ selection as opposed to selection by people. One of them was Alfred Wallace; the other - far better known today - was Charles Darwin. Darwin spent years travelling the world. From 1831 to 1836 he was hired as ship’s naturalist aboard HMS Beagle, and his job was to observe plants and animals and note down what he saw. In a letter of 1877 he says that while on the Beagle he believed in ’the permanence of species’, but on his return home in 1836 he began to think about the deeper meaning of what he had seen, and realized that ’many facts indicated the common descent of species’. By this he meant that species that are different now probably came from ancestors that once belonged to the same species. Species must be able to change. That wasn’t an entirely new idea, but he also came up with an effective mechanism for such changes, and that was new. Meanwhile Wallace was studying the flora and fauna of Brazil and the East Indies, and comparing what he saw in the two regions, and was coming to similar conclusions - and much the same expla­nation. By 1858 Darwin was still mulling over his ideas, contemplating a grand publication of everything he wanted to say about the subject, while Wallace was getting ready to publish a short article containing the main idea. Being a true English gentleman, Wallace warned Darwin of his intentions so that Darwin could pub­lish something first, and Darwin rapidly penned a short paper for the Linnaean Society, followed a year later by a book, The Origin of Species -a big book, but still not on the majestic scale that Darwin had originally intended. Wallace’s paper appeared in the same jour­nal shortly afterwards, but both papers were officially ’presented’ to the Society at the same meeting.

  What was the initial reaction to these two Earth-shattering arti­cles? In his annual report for that year, the President of the Society, Thomas Bell, wrote that ’The year has not, indeed, been marked by any of those striking discoveries which at once revolutionize, so to speak, the department of science in which they occur.’ However, this perception quickly changed as the sheer enormity of Darwin’s and Wallace’s theory began to sink in, and they took a lot of stick from Mustrum Ridcully’s spiritual brethren for daring to come up with a plausible alternative to Biblical creation. What was this epoch-making alternative? An idea so simple that everybody else had missed it. Thomas Huxley is said to have remarked, on reading Origin: ’How extremely stupid not to have thought of that.’

  This is the idea. You don’t need a human being to push animals into new forms; they can do it to themselves ­more precisely: to each other. This was the mechanism of natural selection. Herbert Spencer, who did the important journalistic job of interpreting Darwin’s theory to the masses, coined the phrase, ’survival of the fittest’ to describe it. The phrase had the advantage of convincing everybody that they understood what Darwin was saying, and it had the disadvantage of convincing everybody that they understood what Darwin was saying. It was a classic lie-to-children, and it deceives many critics of evolution to this day, causing them to aim at a long-disowned target, besides giving a spurious ’scientific’ background to some extremely stupid and unpleasant political the­ories.

  Starting from an enormous range of observations of many species of plants and animals, Darwin had become convinced that organisms could change of their own accord, so much so that they could even - over very long periods - change so much that they gave rise to new species.

  Imagine a lot of creatures of the same species. They are in com­petition for resources, such as food - competing with each other, and with animals of other species. Now suppose that by random chance, one or more of these animals has offspring that are better at winning the competition. Then those animals are more likely to survive for long enough to produce the next generation, and the next generation is also better at winning. In contrast, if one or more of these animals has offspring that are worse at winning the com­petition, then those animals are less likely to produce a succeeding generation - and even if they somehow do, that next generation is still worse at winning. Qearly even a tiny advantage will, over many generations, lead to a population composed almost entirely of the new high-powered winners. In fact, the effect of any advantage grows like compound interest, so it doesn’t take all that long. Natural selection sounds like a very straightforward idea, but words like ’competition’ and ’win’ are loaded. It’s easy to get the wrong impression of just how subtle evolution must be. When a baby bird falls out of the nest and gets gobbled up by a passing cat, it is easy to see the battle for survival as being fought between bird and cat. But if that is the competition, then cats are clear winners - so why haven’t birds evolved away altogether? Why aren’t there just cats?

  Because cats and birds long ago came, unwittingly, to a mutual accommodation in which both can survive. If birds could breed unchecked, there would soon be far too many birds for their food supply to support them. A female starling, for instance, lays about 16 eggs in her life. If they all survived, and this continued, the star­ling population would multiply by eight every generation ­16 babies for every two parents. Such ’exponential’ growth is amaz­ingly rapid: by the 70th generation a sphere the size of the solar system would be occupied entirely by starlings (instead of by pigeons, which appears to be its natural destiny).

  The only ’growth rate’ for the population that works is for each breeding pair of adult starlings to produce, on average, exactly one breeding pair of adult starlings. Replacement, but no more - and no less. Anything more than replacement, and the population explodes; anything less, and it eventually dies out. So of those 16 eggs, an average of 14 must not survive to breed. And that’s where the cat comes in, along with all the other things that make it tough to be a bird, especially a young one. In a way, the cats are doing the birds a favour - collectively, though maybe not as individuals. (It depends if you’re one of the two that survive to breed or the 14 that don’t.)

  Rather more obviously, the birds are doing the cats a favour - cat food literally drops out of the skies, manna from heaven. So what stops it getting out of hand is that if a group of greedy cats happens to evolve somewhere, they rapidly eat themselves out of existence again. The more restrained cats next door survive to breed, and quickly take over the vacated territory. So those cats that eat just enough birds to maintain their food supply will win a competition against the greedy cats. Cats and birds aren’t competing because they’re not playing the same game. The real competitions are between cats and other cats, and between birds and other birds. This may seem a wasteful process, but it isn’t. A female starling has no trouble laying her 16 eggs. Life is reproductive ­it makes rea­sonably close, though not exact, copies of itself, in quantity, and ’cheaply’. Evolution can easily ’try out’ many different possibilities, and discard those that don’t work. And that’s an astonishingly effective way to home in on what does work.

  As Huxley said, it’s such an obvious idea. It caused so much trouble from religionists because it takes the gloss off one of their favourite arguments, the argument from design
. Living creatures seem so perfectly put together that surely they must have been designed - and if so, there must have been a Designer. Darwinism made it clear that a process of random, purposeless variation trimmed by self-induced selection can achieve equally impressive results, so there can be the semblance of design without any Designer.

  There are plenty of details to Darwinism that still aren’t under­stood, as with all science, but most of the obvious ways of trying to shoot it down have been answered effectively. The classic example -still routinely trotted out by creationists and others even though Darwin himself had a pretty good answer - is the evolution of the eye. The human eye is a complex structure, and all of its compo­nents have to fit together to a high degree of accuracy, or it won’t work. If we claim that such a complex structure has evolved, we must accept that it evolved gradually. It can’t all have come into being at once. But if so, then at every stage along the evolutionary track the still-evolving proto-eye must offer some kind of survival advantage to the creature that possesses it. How can this happen? The question is often asked in the form ’What use is half an eye?’, to which you are expected to conclude ’nothing’, followed by a rapid conversion to some religion or other. ’Nothing’ is a reasonable answer - but to the wrong question. There are lots of ways to get to an eye gradually that do not require it to be assembled piece by piece like a jigsaw puzzle. Evolution does not build creatures piece by piece like the God of Evolution in The Last Continent. Darwin himself pointed out that in creatures alive in his day you could find all kinds of light-sensitive organs - starting with patches of skin, then increasing in complexity, light-gathering power, and ability to detect fine detail, right up to structures as sophisticated as the human eye. There is a continuum of eyelike organs in the living world, and every creature gains an advantage by having its own type of light-sensing device, in comparison to similar creatures that have a slightly less effective device of a similar kind.